Gas phase dehydration of lactic acid and ethyl lactate to acrylic acid over alkaline-earth phosphates and hydroxyapatites
Résumé
Introduction Acrylic acid (AA) is a platform molecule produced at 4Mt/year by selective oxidation of propene, whose price is growing quickly because of increasing demand and rarefaction of petroleum. An alternative route to produce acrylic acid consists in dehydration of lactic acid (LA) that can be yielded by dehydrogenation of glycerol [1] or direct fermentation of sugars2. However, AA is rarely obtained selectively from LA because of easy decarbonylation leading to acetaldehyde (A) and CO. High yields to AA were obtained using modified zeolites [3], hydroxyapatites (HAP) [4] for LA conversion and calcium phosphates in the case of methyl lactate conversion [5]. In the present work, alkaline-earth pyrophosphates (PP), orthophosphates (OP) and HAP were prepared and evaluated for gas phase dehydration of LA and ethyl lactate (EL). From characterization of structural and acid-base properties, relationships with the catalytic efficiency have been established. Experimental Catalysts were prepared by co-precipitation method. As residual Na was detected for some catalysts prepared using Na containing precursors, Na was added in their labeling. Catalytic testing was carried out in gas phase at several temperatures (300-390°C) and contact times (0.1-10s). They were characterized by XRD, FTIR spectroscopy, BET measurements and ICP analysis. The surface characterization was obtained by both XPS and 1H-31P CP-MAS NMR. Acid-Base proprieties were measured by NH3/CO2-TPD respectively and pyridine adsorption followed by FTIR. Results and Discussion Good performances were obtained for LA conversion for different OP and PP catalysts with selectivity to AA reaching 50% near complete conversion. The main by-product was A (30%) limiting enhancement of LA dehydration. Catalysts were stable for at least 24 hours and only small amounts of coke were evidenced after reaction. The optimized temperature of 380°C was shown to favor AA desorption without thermal degradation. Close performances were obtained with different structures which can be explained by kinetic limitations or similar surface species. Surface characterization revealed the presence of a hydrogenated phosphate phase different from the bulk. Conversion of EL strongly inhibited the decarbonylation pathway (A selectivity of 4-10%) enhancing the dehydration pathway (AA+EA selectivity of 87%) although lower activities were observed. Decreasing contact time enhanced the AA selectivity without affecting the EA one indicating that two pathways took place on two different active sites: direct dehydration and simultaneous hydrolysis-dehydration of EL. HAP were less selective for LA dehydration but, in the case of EL dehydration, they showed to be promising systems since higher yields to valuable products such as AA and EA were achieved. AL and EL conversions showed that different active sites had to be considered, probably because in LA conversion, the vapor partial pressure of the feed was high (66%) contrarily to EL conversion (0%). The NH3 and CO2 TPD profiles were similar with only weak and moderated strength sites. It suggested that high proportion of acid-base pairs were present at the surface as confirmed by the low acid to base balances (0.9-2.1). For AL conversion, correlation between the AA selectivity and this balance was established, while for EL conversion, the catalysts with the weakest acid and base strengths exhibit the best performances without excluding that the nature of sites (Lewis and Brřnsted) might also influence the catalytic performances.